A couple times I had a few tubes that you could not see or read any numbers. I wrapped them in a couple paper towels and put them in the freezer for a few hours. Then get your reading glasses or magnifier and good lighting ready, pull the tube out, unwrap and carefully look. It might take a little while for the numbers to be readable. After the tube is id'd I write the number on the base with a paint pen.

I have been told the tube number is directly in line with the key at the base of the tube.Example, take octal tube, set the key directly in front of you, the tube number I directly above that key.

Some brands did this, others didn't. Sometimes the number is on the base; these don't usually disappear. Sometimes it is on the top of the tube; this is especially true of Loctals and Compactrons. The rule is, if you don't see it or the remains of it where you are looking, look somewhere else.

I've had good luck with shinning a strong flashlight kinda sideways to where the numbers would be.. And moving the tube around until I find what i'm looking for.. Incandescent light seems to work better for me.. Yeah then paint pen it on the base..

A little oil off your nose , brushed on with your finger in different directions will usually reveal the type. Look at it under your magnifier lamp. Concentrate on the area "above the key" most often its there.Mark Oppat

Denis wrote: "Super glue fumes will sometimes work." That may also reveal fingerprints of anyone that had touched the tube. Or enough time sniffing the ca fumes up-close trying to read the tube type, I may not care what the tube is....ha.